# std::bind_front, std::bind_back

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< cpp‎ | utility‎ | functional

Defined in header `<functional>` |  |
`std::bind_front` |  |
template< class F, class... Args > constexpr /* unspecified */ bind_front( F&& f, Args&&... args ); | (1) | (since C++20)
template< auto ConstFn, class... Args > constexpr /* unspecified */ bind_front( Args&&... args ); | (2) | (since C++26)
`std::bind_back` |  |
template< class F, class... Args > constexpr /* unspecified */ bind_back( F&& f, Args&&... args ); | (3) | (since C++23)
template< auto ConstFn, class... Args > constexpr /* unspecified */ bind_back( Args&&... args ); | (4) | (since C++26)

Function templates `std::bind_front` and `std::bind_back` generate a perfect forwarding call wrapper which allows to invoke the callable target with its (1,2) first or (3,4) last sizeof...(Args) parameters bound to args.

1,3) The call wrapper holds a copy of the target callable object f.
2,4) The call wrapper does not hold a callable target (it is statically determined).
1) std::bind_front(f, bound_args...)(call_args...) is expression-equivalent to
std::invoke(f, bound_args..., call_args...).
2) std::bind_front<ConstFn>(bound_args...)(call_args...) is expression-equivalent to
std::invoke(ConstFn, bound_args..., call_args...).
3) std::bind_back(f, bound_args...)(call_args...) is expression-equivalent to
std::invoke(f, call_args..., bound_args...).
4) std::bind_back<ConstFn>(bound_args...)(call_args...) is expression-equivalent to
std::invoke(ConstFn, call_args..., bound_args...).

The following conditions must be true, otherwise the program is ill-formed:

- (1,3) std::is_constructible_v<std::decay_t<F>, F>,
- (1,3) std::is_move_constructible_v<std::decay_t<F>>,
- (2,4) If decltype(ConstFn) is a pointer or a pointer-to-member then `ConstFn` is not a null pointer,
- (std::is_constructible_v<std::decay_t<Args>, Args> && ...),
- (std::is_move_constructible_v<std::decay_t<Args>> && ...).

### Parameters

f | - | Callable object (function object, pointer to function, reference to function, pointer to member function, or pointer to data member) that will be bound to some arguments
args | - | list of the arguments to bind to the (1,2) first or (3,4) last sizeof...(Args) parameters of the callable target
Type requirements
- `std::decay_t<F>` must meet the requirements of MoveConstructible.
- `std::decay_t<Args>...` must meet the requirements of MoveConstructible.
- `decltype(ConstFn)` must meet the requirements of Callable.

### Return value

A function object (the call wrapper) of type `T` that is unspecified, except that the types of objects returned by two calls to `std::bind_front` or `std::bind_back` with the same arguments are the same.

Let `bind-partial` be either `std::bind_front` or `std::bind_back`.

The returned object has the following properties:

## bind-partial return type

#### Member objects

The returned object behaves as if it holds:

1,3) A member object `fd` of type std::decay_t<F> direct-non-list-initialized from std::forward<F>(f), and
1-4) An std::tuple object `tup` constructed with std::tuple<std::decay_t<Args>...>(std::forward<Args>(args)...), except that the returned object's assignment behavior is unspecified and the names are for exposition only.

#### Constructors

The return type of `bind-partial` behaves as if its copy/move constructors perform a memberwise copy/move. It is CopyConstructible if all of its member objects (specified above) are CopyConstructible, and is MoveConstructible otherwise.

#### Member function `operator()`

Given an object `G` obtained from an earlier call to (1,3) `bind-partial(f, args...)` or (2,4) `bind-partial<ConstFn>(args...)`, when a glvalue `g` designating `G` is invoked in a function call expression g(call_args...), an invocation of the stored object takes place, as if by:

1) std::invoke(g.fd, std::get<Ns>(g.tup)..., call_args...), when `bind-partial` is `std::bind_front`,
2) std::invoke(ConstFn, std::get<Ns>(g.tup)..., call_args...), when `bind-partial` is `std::bind_front`,
3) std::invoke(g.fd, call_args..., std::get<Ns>(g.tup)...), when `bind-partial` is `std::bind_back`,
4) std::invoke(ConstFn, call_args..., std::get<Ns>(g.tup)...), when `bind-partial` is `std::bind_back`,

where

  - `Ns` is an integer pack `0, 1, ..., (sizeof...(Args) - 1)`,
  - `g` is an lvalue in the std::invoke expression if it is an lvalue in the call expression, and is an rvalue otherwise. Thus std::move(g)(call_args...) can move the bound arguments into the call, where g(call_args...) would copy.

The program is ill-formed if `g` has volatile-qualified type.

The member operator() is `noexcept` if the std::invoke expression it calls is noexcept (in other words, it preserves the exception specification of the underlying call operator).

### Exceptions

1,3) Throw any exception thrown by calling the constructor of the stored function object.
1-4) Throw any exception thrown by calling the constructor of any of the bound arguments.

### Notes

These function templates are intended to replace std::bind. Unlike `std::bind`, they do not support arbitrary argument rearrangement and have no special treatment for nested bind-expressions or std::reference_wrappers. On the other hand, they pay attention to the value category of the call wrapper object and propagate exception specification of the underlying call operator.

As described in std::invoke, when invoking a pointer to non-static member function or pointer to non-static data member, the first argument has to be a reference or pointer (including, possibly, smart pointer such as std::shared_ptr and std::unique_ptr) to an object whose member will be accessed.

The arguments to `std::bind_front` or `std::bind_back` are copied or moved, and are never passed by reference unless wrapped in std::ref or std::cref.

Typically, binding arguments to a function or a member function using (1) `std::bind_front` and (3) `std::bind_back` requires storing a function pointer along with the arguments, even though the language knows precisely which function to call without a need to dereference the pointer. To guarantee "zero cost" in those cases, C++26 introduces the versions (2,4) (that accept the callable object as an argument for non-type template parameter).

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_bind_front` | `201907L` | (C++20) | `std::bind_front`, (1)
`202306L` | (C++26) | Allow passing callable objects as non-type template arguments to `std::bind_front`, (2)
`__cpp_lib_bind_back` | `202202L` | (C++23) | `std::bind_back`, (3)
`202306L` | (C++26) | Allow passing callable objects as non-type template arguments to `std::bind_back`, (4)

### Possible implementation

(2) bind_front
    namespace detail
    {
        template<class T, class U>
        struct copy_const
            : std::conditional<std::is_const_v<T>, U const, U> {};

        template<class T, class U,
                 class X = typename copy_const<std::remove_reference_t<T>, U>::type>
        struct copy_value_category
            : std::conditional<std::is_lvalue_reference_v<T&&>, X&, X&&> {};

        template <class T, class U>
        struct type_forward_like
            : copy_value_category<T, std::remove_reference_t<U>> {};

        template <class T, class U>
        using type_forward_like_t = typename type_forward_like<T, U>::type;
    }

    template<auto ConstFn, class... Args>
    constexpr auto bind_front(Args&&... args)
    {
        using F = decltype(ConstFn);

        if constexpr (std::is_pointer_v<F> or std::is_member_pointer_v<F>)
            static_assert(ConstFn != nullptr);

        return
            [... bound_args(std::forward<Args>(args))]<class Self, class... T>
            (
                this Self&&, T&&... call_args
            )
            noexcept
            (
                std::is_nothrow_invocable_v<F,
                    detail::type_forward_like_t<Self, std::decay_t<Args>>..., T...>
            )
            -> std::invoke_result_t<F,
                    detail::type_forward_like_t<Self, std::decay_t<Args>>..., T...>
            {
                return std::invoke(ConstFn, std::forward_like<Self>(bound_args)...,
                                   std::forward<T>(call_args)...);
            };
    }
(4) bind_back
    namespace detail { /* is the same as above */ }

    template<auto ConstFn, class... Args>
    constexpr auto bind_back(Args&&... args)
    {
        using F = decltype(ConstFn);

        if constexpr (std::is_pointer_v<F> or std::is_member_pointer_v<F>)
            static_assert(ConstFn != nullptr);

        return
            [... bound_args(std::forward<Args>(args))]<class Self, class... T>
            (
                this Self&&, T&&... call_args
            )
            noexcept
            (
                std::is_nothrow_invocable_v<F,
                    detail::type_forward_like_t<Self, T..., std::decay_t<Args>>...>
            )
            -> std::invoke_result_t<F,
                    detail::type_forward_like_t<Self, T..., std::decay_t<Args>>...>
            {
                return std::invoke(ConstFn, std::forward<T>(call_args)...,
                                   std::forward_like<Self>(bound_args)...);
            };
    }

### Example

Run this code

    #include <cassert>
    #include <functional>

    int minus(int a, int b)
    {
        return a - b;
    }

    struct S
    {
        int val;
        int minus(int arg) const noexcept { return val - arg; }
    };

    int main()
    {
        auto fifty_minus = std::bind_front(minus, 50);
        assert(fifty_minus(3) == 47); // equivalent to: minus(50, 3) == 47

        auto member_minus = std::bind_front(&S::minus, S{50});
        assert(member_minus(3) == 47); //: S tmp{50}; tmp.minus(3) == 47

        // Noexcept-specification is preserved:
        static_assert(!noexcept(fifty_minus(3)));
        static_assert(noexcept(member_minus(3)));

        // Binding of a lambda:
        auto plus = [](int a, int b) { return a + b; };
        auto forty_plus = std::bind_front(plus, 40);
        assert(forty_plus(7) == 47); // equivalent to: plus(40, 7) == 47

    #if __cpp_lib_bind_front >= 202306L
        auto fifty_minus_cpp26 = std::bind_front<minus>(50);
        assert(fifty_minus_cpp26(3) == 47);

        auto member_minus_cpp26 = std::bind_front<&S::minus>(S{50});
        assert(member_minus_cpp26(3) == 47);

        auto forty_plus_cpp26 = std::bind_front<plus>(40);
        assert(forty_plus(7) == 47);
    #endif

    #if __cpp_lib_bind_back >= 202202L
        auto madd = [](int a, int b, int c) { return a * b + c; };
        auto mul_plus_seven = std::bind_back(madd, 7);
        assert(mul_plus_seven(4, 10) == 47); //: madd(4, 10, 7) == 47
    #endif

    #if __cpp_lib_bind_back >= 202306L
        auto mul_plus_seven_cpp26 = std::bind_back<madd>(7);
        assert(mul_plus_seven_cpp26(4, 10) == 47);
    #endif
    }

### References

- C++26 standard (ISO/IEC 14882:2026):

  - TBD Function templates bind_front and bind_back [func.bind.partial]

- C++23 standard (ISO/IEC 14882:2024):

  - 22.10.14 Function templates bind_front and bind_back [func.bind.partial]

- C++20 standard (ISO/IEC 14882:2020):

  - 20.14.14 Function template bind_front [func.bind.front]

### See also

bind (C++11) | binds one or more arguments to a function object (function template)
mem_fn (C++11) | creates a function object out of a pointer to a member (function template)

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